1. Field of the Invention
The invention relates generally to the field of wellbore instruments and well logging methods. More specifically, the invention relates to systems and methods for operating electrically powered instruments in a well using a wired pipe string as a signal communication channel.
2. Background Art
Well logging instruments are devices configured to move through a wellbore drilled through subsurface rock formations. The devices include one or more sensors and other devices that measure various properties of the subsurface rock formations and/or perform certain mechanical acts on the formations, such as drilling or percussively obtaining samples of the rock formations, and withdrawing samples of connate fluid from the rock formations. Measurements of the properties of the rock formations made by the sensors may be recorded with respect to the instrument axial position (depth) within the wellbore as the instrument is moved along the wellbore. Such recording is referred to as a “well log.”
Well logging instruments can be conveyed along the wellbore by extending and withdrawing an armored electrical cable (“wireline”), wherein the instruments are coupled to the end of the wireline. Such conveyance relies on gravity to move the instruments into the wellbore. Extending and withdrawing the wireline may be performed using a winch or similar spooling device known in the art. However, gravity can only be used on substantially vertical wellbores. Those deviating from vertical require additional force to move through the wellbore.
There are several types of wireline instrument conveyance known in the art for the foregoing conditions. One conveyance technique includes coupling the wireline instruments to the end of a coiled tubing having a wireline disposed therein. The wireline instruments are extended into and withdrawn from the wellbore by extending and retracting the coiled tubing, respectively. A subset of such coiled tubing techniques includes preliminary conveyance of the wireline configurable well logging instruments to a selected depth in the wellbore using a threadedly coupled pipe “string.” See, for example, U.S. Pat. No. 5,433,276 issued to Martain et al. However, the use of coiled tubing with wireline instruments is costly and is inherently limited by the amount of pushing force capable with the coiled tubing. As a result, the use of coiled tubing is typically problematic in extended reach wells.
Another well logging instrument conveyance technique includes coupling wireline configurable well logging instruments to the end of a drill pipe or similar threadedly coupled pipe string. A wireline is coupled to the instruments using a “side entry sub” which provides a sealable passage from the exterior of the pipe string to the interior thereof. As the pipe string is extended into the wellbore, the wireline is extended by operating a conventional winch. An example of the foregoing is described in U.S. Pat. No. 6,092,416 issued to Halford et al. and assigned to the assignee of the present invention. However, this conveyance technique is frequently unreliable as the wireline is positioned in the annulus and subject to crushing, splicing or other damage. For example, the wireline may become pinched between the drill pipe and the casing or wellbore.
Additionally, the well logging instruments may be positioned at the end of a drill pipe without use of a wireline cable. In such circumstances, each well logging instrument is provided with a battery and memory to store the acquired data. As a result, the well logging instruments cannot communicate with the surface while downhole. In addition, the data acquired cannot be analyzed at the surface until the wireline instruments return to the surface. Without any communication with the surface, surface operators cannot be certain the instruments are operating correctly, cannot control the instruments while downhole, and the data cannot be analyzed until after the wireline instruments are removed from the wellbore.
Recently, a type of drill pipe has been developed that includes a signal communication channel. See, for example, U.S. Pat. No. 6,641,434 issued to Boyle et al. and assigned to the assignee of the present invention. Such drill pipe, known as wired drill pipe, has in particular provided substantially increased signal telemetry speed for use with LWD instruments over conventional LWD signal telemetry, which typically is performed by mud pressure modulation or by very low frequency electromagnetic signal transmission.
The foregoing wired drill pipe having a signal communication channel has not proven effective at transmitting electrical power from the surface to an instrument string disposed at a lower end of the pipe. In wireline conveyance of wellbore instrument, electrical power is transmitted from the surface to the instruments in the wellbore using one ore more insulated electrical conductors in the wireline cable. In MWD and LWD, electrical power may be provided by batteries, or by an electric generator operated by flow of fluid through the pipe. When wired pipe is used for signal telemetry, the amount of electrical power required by the instruments may be substantially reduced because the signal telemetry device used in MWD/LWD, typically a mud flow modulator, uses a substantial portion of the total electrical power used by the instruments in the bottom hole assembly.
What is needed is a system and method for pipe conveyance of wellbore instruments that includes substantial signal telemetry capability, and does not require the use of armored electrical cable for continuous transmission of electrical power to the instruments in the wellbore or signal communication from the instruments to the surface.